Communication device having relaying and switching function

ABSTRACT

In a communication device having a transmission device connected to first and second transmission paths, and an exchange connected to the transmission device, the transmission device includes a codec and a switching part. When a speech signal received from the first transmission path terminates at a terminal accommodated in the exchange, the speech signal having a low bit rate is converted, by the codec, into a bit rate which can be processed by the exchange, and then sent to the exchange. Meanwhile, when the speech signal received from the first transmission path is relayed to the second transmission path, the speech signal having the low bit rate is sent to the second transmission path via the switching part without passing it through the codec while the bit rate of the speech signal is maintained.

TECHNICAL FIELD

The present invention relates to a communication system which compressessignals, such as speech signals and multiplexes these signals with eachother to send these signals to a high-bit-rate digital line in the formof a multiplexed signal. More specifically, the present invention isconcerned with a communication device having a function of terminating areceived call at a terminal, or relaying it to the high-bit-rate digitalline.

BACKGROUND ART

Recently, there has been in practical use a communication system whichsends to a high-bit-rate digital line a signal obtained by compressingand multiplexing speech signals. Conventionally, speech signals aremultiplexed at a bit rate of 64 Kbps, and a multiplexed signal thusobtained is sent to a high-bit-rate digital line. Further, recentlythere has also been provided a system in which speech signals have beenmultiplexed at a bit rate of 32 Kbps.

FIG. 1 shows a conventional communication system having a function ofcompressing and multiplexing speech signals in order to send thesesignals to a high-bit-rate digital line. The communication system shownin FIG. 1 functions as a WAN (Wide Area Network), and comprisesexchanges 10A, 10B and 10C, such as PBXs (Private Branch Exchanges),transmission devices 20A, 20B and 20C, and transmission paths L1 and L2which are high-bit-rate digital lines. The transmission path L1 connectsthe transmission devices 20A and 20B to each other, and the transmissionline L2 connects the transmission devices 20B and 20C to each other.

Each of the exchanges 10A, 10B and 10C accommodates terminals T1, T2 andT3, such as telephone sets, and terminals other than telephone sets,such as data terminals and facsimile machines A circuit of acommunication path system of each of the exchanges 10A, 10B and 10Cswitches 64 Kbps signals. Hence, each of the exchanges 10A, 10B and 10Chas a PCM (Pulse Code Modlation) converter which converts analog signalsfrom the terminals T1, T2 and T3 into a PCM speech signal having a bitrate of 64 Kbps and executes the reverse operation. It may be possibleto provide the above PCM converter for each analog terminal.

The transmission device 20A connected to the exchange 10A comprises acompressor 21, an expander 22 and a multiplexing circuit 23. Thecompressor 21 converts the 64 Kbps PCM speech signal into an ADPCM(Adaptive Differential Pulse Code Modulation) signal having a bit rateof 32 Kbps. As shown, the compressor 21 is comprised of an adaptivequantizer 21a, a predictor 21b and a difference device 21c. Generally, aplurality of compressors 21 are provided. The multiplexing circuit 23multiplexes the 32 Kbps ADPCM signals with other digital signals (imagesignals and data signals), and sends a multiplexed signal to thetransmission path L1. Further, the multiplexing circuit 23 separates aplurality of digital signals from a multiplexed signal received from thetransmission line L1. The expander 22 demodulates the received 32 KbpsADPCM signal to generate a 64 Kbps PCM speech signal. The demodulatedPCM speech signal is sent to the exchange 10A. The compressor 21 and theexpander 22 form a speech codec. The transmission device 20C and theexchange 10C connected thereto are respectively configured in the samemanner as the above transmission device 20A and the exchange 10A.

The transmission device 20B comprises a multiplexing circuit 24connected to the transmission line L1, and a speech codec 25 connectedthereto. Further, the transmission device 20B comprises a multiplexingcircuit 27 connected to the transmission line L2, and a speech codec 26connected thereto. Each of the speech codecs 25 and 26 has the functionsof both the compressor 21 and the expander 22.

It will now be assumed that the terminal T1 accommodated in the exchange10A generates a call requesting to communicate with the terminalaccommodated in the exchange 10B, a speech signal from the terminal T1being converted into a 64 Kbps PCM speech signal in the exchange 10A,which is then converted into a 32 Kbps ADPCM speech signal by thecompressor 21 of the transmission device 20A. The ADPCM signal ismultiplexed with other signals, and then sent to the transmission pathL1. The multiplexing device of the transmission device 20B demultiplexesthe received multiplexed signal to generate separate signals. The codec25 converts the 32 Kbps ADPCM signal into the 64 Kbps PCM speech signal.The exchange 10B discriminates a dial signal in a control channelreceived via the multiplexing device 24, and recognizes that the callfrom the terminal T1 should terminate at the terminal T2. The exchange10B specifies a channel coupling the terminals T1 and T2 to each other,and then they become connected to each other.

In a case where the terminal T1 calls the terminal T3 accommodated inthe exchange 10C, the exchange 10B analyzes a dial signal from theterminal T1 and recognizes that the call should be sent to the exchange10C. In this case, the exchange 10C sets a channel connecting thetransmission paths L1 and L2 to each other. A signal received from thetransmission path L1 via the multiplexing circuit 24 and the speechcodec 25 is sent to the transmission path L2 via the exchange 10B, thespeech codec 26 and the multiplexing circuit 27. During this time, the64 Kbps PCM speech signal from the exchange 10B is converted into the 32Kbps ADPCM signal by the speech codec 26. Then, the speech signal fromthe terminal T1 is supplied to the terminal via the transmission device20C and the exchange 10C. A speech signal from the terminal T3 istransmitted to the terminal T1 in a route the reverse of the above route

As described above, in the communication system, the compression andexpansion processes are repeatedly carried out. The above bit-rateconversion (64 Kbps→32 Kbps) does not cause a deterioration of speech interms of the principle. The compression and expansion process using theADPCM is particularly called transcoding. That is, on the transmissionside, the next sampling value is predicted from a sampling value, andonly the difference between the predicted value and the real value isquantized and transmitted. On the receiving side, a predicted value isadded to the transmitted residual signal to reproduce the real value. Onthe transmission side, the quantizing step is changed so that thedifference between the predicted value and the real value becomessmaller. By using the above transcoding technique, the quality of the 32Kbps speech signal on the transmission path L1 is theoretically the sameas that of the 32 Kbps speech signal on the transmission path L2 evenwhen the above compression and expansion processes are repeatedlycarried out. As a result, there is no deterioration of speech even whenthe speech signal is repeatedly relayed and transmitted.

However, a problem will occur when the transmission paths L1 and L2transmit further compressed signals, such as 16 Kbps or 8 Kbps signals.Generally, in a speech signal up to a low bit rate lower than or equalto 16 Kbps, it is very difficult to apply the transcoding technique usedin the 32 Kbps ADPCM. That is, the quality of speech will greatlydeteriorate if the low-bit-rate signals having a bit rate lower than orequal to 16 Kbps are processed by the transcoding technique.

In a conventional technique, a process shown in FIG. 2 is used in orderto transmit the speech signal at a low bit rate lower than or equal to16 Kbps. In the transmission device 20B, surplus bits (dummy bits)amounting to 48 Kbps (56 Kbps) are added to the 16 Kbps (8 Kbps) speechsignal received from the transmission path L1, so that a 64 Kbps signalis generated a shown in FIG. 2). The 64 Kbps signal containing the dummybits is switched via a time switch TS without being changed, and sent tothe transmission device 20B b in FIG. 2). The transmission device 20Bremoves the dummy bits from the received 64 Kbps signal and reproducesthe 16 Kbps (8 Kbps) speech signal. When the 16 Kbps (8 Kbps) signalshould be only switched for relay, it is sent to an objective path(transmission path L2 in this case) c in FIG. 2). On the other hand,when the 16 Kbps (8 Kbps) signal is a call requested to terminate at theterminal T2, the 16 Kbps (8 Kbps) speech signal is output to the speechcodec having the 16 Kbps→64 Kbps conversion function in the transmissiondevice 20B (c in FIG. 2). The codec converts only the 16 Kbps speechsignal into a 64 Kbps speech signal, and outputs the 64 Kbps speechsignal to a line switch LS in the exchange 10B (d in FIG. 2). The lineswitch LS outputs the 64 Kbps speech signal to the terminal T2.

However, the structure shown in FIG. 2 has a disadvantage in that theefficiency of processing is poor and it takes a long time to process thesignals because irrespective of whether or not each call should berelayed to the transmission path L2 or terminate at a terminalaccommodated in the exchange 10B, the dummy bits are added to all inputspeech signals and removed there from and thereafter the 16 Kbps (8Kbps) speech signals are converted into the 64 Kbps signals which can beprocessed by the exchange 10B.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a communication devicecapable of efficiently executing the relay process and the terminatingprocess within a short time without deteriorating the quality of speechin a relaying and switching system having a transmission path via whicha low-bit-rate compressed speech signal is transmitted.

The above object of the present invention is achieved by a communicationdevice including a transmission device connected to first and secondtransmission paths, and an exchange connected to the transmissiondevice, the communication device being configured as follows. Theexchange comprises: first control means for determining whether an inputsignal which has a first bit rate and is received by the transmissiondevice via the first transmission path is a first signal which should beone of a plurality of terminals connected to the exchange or a secondsignal which should be relayed to the second transmission path via thetransmission device; and first switching means for selectivelyconnecting the transmission device and the terminals and transferring asignal having a second bit rate between them. The transmission devicecomprises: second switching means for selectively connecting the firstand second transmission paths and the exchange and transferring a signalhaving the first bit rate between them; converting means for convertingthe input signal having the first bit rate into a signal having thesecond bit rate which is output to the exchange and for converting asignal which has the second bit rate and is received via the exchangeinto a signal having the first bit rate which is output to the secondswitching means; and second control means for controlling, when thefirst control means determines that the input signal is the secondsignal, the second switching means so that the input signal is output tothe exchange via the conversion means and for controlling, when thefirst control means determines that the input signal is the firstsignal, the second switching means so that the input signal is output tothe second transmission path without changing a bit rate thereof.

The above object of the present invention is also achieved by acommunication device including a transmission device connected to firstand second transmission paths, and an exchange connected to thetransmission device, the communication device being configured asfollows. The transmission device comprises: codec means for convertingan input signal which has a first bit rate and is received via the firsttransmission path into a first converted input signal having a secondbit rate and for converting an output signal which has the second bitrate and is received from the exchange into the first converted outputsignal having the first bit rate; dummy bit inserting and dropping meansfor adding dummy bits to the input signal so that a second convertedinput signal having the second bit rate is generated and for removingthe dummy bits from a dummy-bit added output signal received from theexchange so that the second converted output signal having the first bitrate is generated; and selecting means for selecting either the codecmeans or the dummy bit inserting and dropping means in accordance with acontrol signal.

The exchange comprises: first control means for determining whether theinput signal which has the first bit rate and is received by thetransmission device via the first transmission path is a first signalwhich should terminate at one of a plurality of terminals connected tothe exchange or a second signal which should be relayed to the secondtransmission path; and switching means for selectively connecting thetransmission device and the terminals to transfer signals respectivelyhaving the second bit rate.

Further, the transmission device comprises second means for making theselect means select the codec means when the first control meansdetermines that the input signal is the first signal and for making theselect means select the dummy bit inserting and dropping means when thefirst control means determines that the input signal is the secondsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a communication system using conventionalcommunication devices;

FIG. 2 is a block diagram of another conventional communication deviceintended to eliminate disadvantages of the conventional communicationdevices shown in FIG. 1;

FIG. 3 is a block diagram of a communication system in whichcommunication devices of the present invention are used;

FIG. 4 is a block diagram of a communication device according to a firstembodiment of the present invention;

FIG. 5 is a flowchart of the operation of the communication device shownin FIG. 4;

FIG. 6 is a block diagram of a communication device different from thecommunication device shown in FIG. 4; and

FIG. 7 is a block diagram of a transmission device in a communicationdevice according to a second embodiment of the present invention.

BEST MODE OF CARRYING OUT THE INVENTION

FIG. 3 is a block diagram of a communication system in which the presentinvention is used. The communication system shown in FIG. 3 comprisesexchanges 110A, 110B and 110C, and transmission devices 120A, 120B and120C. Each of the exchanges 110A, 110B and 110C operates at a bit rateof 64 Kbps. Each of the transmission devices 120A, 120B and 120Ccomprises a speech codec which compresses a 64 Kbps speech signal into asignal having a bit rate lower than or equal to 16 Kbps and whichconverts a speech signal having a bit rate lower than or equal to. 16Kbps into a 64 Kbps speech signal, and a multiplexing circuit whichgenerates a multiplexed signal by multiplixing signals, such as 16 Kbpsspeech signals, and which generates signals, such as 16 Kbps speechsignals, by demultiplexing a multiplexed signal. As will be describedlater, the transmission device 120B has a relaying and switchingfunction. The transmission paths L1 and L2 transmit speech signals andother signals respectively having bit rates lower than or equal to 16Kbps. It will now be assumed that hereinafter, the low bit rate is equalto 16 Kbps.

FIG. 4 shows the exchange 110B and the transmission device 120B shown inFIG. 3 in more detail. The exchange 110B comprises a control part 111and a switching part 112. The exchange 110B executes a switchingoperation under the control of the control part 111. The transmissiondevice 120B comprises a control part 121, a switching part 122, a speechcodec 123, and multiplexing circuits 124 and 127. The multiplexingcircuit 124 has the function of separating 16 Kbps speech signals andother signals from a multiplexed signal received via the transmissionpath L1 and outputting these signals to the switching part 122, and thefunction of multiplexing 16 Kbps speech signals and other signals witheach other and sending a multiplexed signal to the transmission path L1.The multiplexing circuit 127 separates 16 Kbps speech signals and othersignals from a multiplexed signal received via the transmission path L2and outputting these signals to the switching part 122, and the functionof multiplexing 16 Kbps speech signals and other signals with each otherand sending a multiplexed signal to the transmission path L2. The speechcodec 123 has the function of converting only the 16 Kbps speech signalsfrom the switching part 122 into a 64 Kbps PCM speech signal and sendingit to the exchange 110B. The speech codec 123 has the function ofconverting a 64 Kbps PCM signal from the exchange 110B into 16 Kbpsspeech signals and sending these signals to the switching part 122. Theswitching part 122 executes a switching operation under the control ofthe control part 121, as will be described later. For example, when acall received via the transmission path L1 should be sent to thetransmission L2 by the relaying and switching operation, the exchange122 relays the signal from the transmission path L1 to the transmissionpath L2. When the switching part 122 comprises a time switch, the signalis relayed by specifying a time slot (channel) on an input highway and atime slot (channel) on an output highway. The multiplexing circuit 124and the control part 111 of the exchange 110B are connected to eachother via a control line 131. Similarly, the multiplexing circuit 127and the control part 111 are connected to each other via a control line132. The multiplexing circuit 124 sends to the transmission line L1 avariety of control information from the control part 111 correspondingto D-channel information set forth in the CCITT Recommendations andreceives such control information from the transmission line L1.Similarly, the multiplexing circuit 127 sends a variety of controlinformation to the transmission line L2 and receives such controlinformation from the transmission line L2. The control part 121 of thetransmission device 120B and the control part 111 of the exchange 110Bare connected to each other via a control line 133. The speech codec 123and the switching part 112 of the exchange 110B are connected to eachother via a signal line 134.

A description will now be given, with reference to an operationflowchart of FIG. 5, of the operation of the relaying and switchingdevice shown in FIG. 4. Assuming that the terminal T1 shown in FIG. 4generates a call, the control part 111 of the exchange 110B receives thecall from the exchange 110A via the multiplexing circuit 124 and thecontrol line 131 (step S11). The control part 111 of the exchange 110Bdetermines, from discrimination information, such as a dial number,whether the call should terminate at a terminal accommodated by theexchange 110B (terminal T2, for example) or should be switched andrelayed to the transmission path L2 (step S12). When it is determinedthat the received call should terminate at a terminal accommodated bythe exchange 110B, the controller 111 of the exchange 110B sends thecontrol part 121 of the transmission apparatus 120B an instruction forconnecting the channel carrying the call and an idle channel in thespeech codec 123 to each other via the switching part 122. The speechcodec 123 converts the 16 Kbps speech signals into the 64 Kbps speechsignal, and outputs the 64 Kbps speech signal to the switching part 112of the exchange 110 via the signal line 134. The switching part 112outputs the 64 Kbps speech signal from the speech codec 123 to theterminal T2 under the control of the controller 111.

When it is determined, in step S12, that the call received in step S12should be relayed to the transmission path L2, the control part 111sends relay information (which shows that the called terminal is locatedon the transmission path L2) to the control part 121 of the transmissiondevice 120B via the control line 133 (step S14). The control part 121receives the relay information and connects the channel carrying thecall on the side of the multiplexing circuit 124 and an idle channel onthe side of the multiplexing circuit 127 (step S15). After theconnection has been completely made, the control part 121 informs thecontrol part 111 of the exchange 110B of the completion of the relayoperation (step S16). In response to this completion of the relayoperation, the control part 111 sends control information (connectioninformation) about the call from the terminal T1 to the transmissionpath L2 via the control line 132 and the multiplexing circuit 127. The16 Kbps speech signal from the terminal T1 is relayed via the switchingpart 122 without being changed.

The switching part 122 has the function of switching all the datachannels of the transmission paths L1 and L2. In this regard, theswitching part 122 is similar to the switching part 112 of the exchange110B. However, the switching part 112 has various service functions andthe dial recognition function, while the switching part 122 of thetransmission device 120B does not need such functions and has a smallsize. Normally, the transmission path L1 is coupled to the exchange 110Bvia the speech codec 123 as long as the relay instruction from theexchange 110B has not been received.

As described above, according to the first embodiment of the presentinvention, a call which should be relayed is processed by thetransmission device 120B without the exchange 110B. That is, it is notnecessary to repeatedly carry our the compression and expansion. Hence,it becomes possible to efficiently transmit the speech signals via alow-bit-rate transmission path at a high speed without deteriorating thequality of speech.

FIG. 6 shows another structure of the transmission device 120B. Insteadof the time switch, the transmission device 120B uses a multiplexing busin order to realize the switching operation. The transmission device120B shown in FIG. 6 comprises line interfaces 141 and 142, amultiplexing bus 143, a control bus 144, a control part 145 and a speechcodec 146. The line interface 141 couples the multiplexing bus 143 withthe transmission path L1 carrying low-bit-rate signals, such as 16 Kbpssignals, with each other. The line interface 142 couples themultiplexing bus 143 with the transmission path L2 carrying amultiplexed signal obtained by multiplexing low-bit-rate signals, suchas 16 Kbps signals, with each other. Control information necessary forthe switching operation is transferred between the line interfaces 141and 142 and the controller 145 via the control bus 144. The multiplexingbus 143 multiplies real data, such as speech signals, with each other.The speech codec 146 has the function of converting 16 Kbps speechsignals from the multiplexing bus 143 into a 64 Kbps speech signal andthe function of converting a 64 Kbps speech signal from the signal line134 into 16 Kbps speech signals, which are sent to the multiplexing bus143. The control part 145 and the exchange 110B shown in FIG. 3 areconnected to each other via the control line 133.

The operation of the transmission device shown in FIG. 6 will now bedescribed. Normally, a signal from the transmission path L1 is coupledto the speech codec 146 using time slot #1 on the multiplexing bus 143(hereinafter referred to as time slot TS1). A signal from thetransmission path L2 is coupled to the speech codec 146 using time slot#2 (hereinafter referred to as time slot TS2). It will now be assumedthat a call from the transmission path L1 terminates at the terminal T2accommodated in the exchange 110B connected to the transmission device120B. In this case, the control part 111 (FIG. 4) of the exchange 110Breceives control information via the line interface 141, the control bus144, the control part 145 and the control line 133, and therebyrecognizes that there is a call which should terminate at the terminalT2. The control part 111 shown in FIG. 4 controls the switching part 112so that the 64 Kbps speech signal received from the speech codec 146 viathe signal line 134 is output to the terminal T2.

Meanwhile, when a call from the transmission line L1 should be relayedto the transmission path L2, the control part 111 of the exchange 110Breceiving the control information via the above-mentioned route outputsthe relay instruction to the control part 145 of the transmission device120B. In response to the relay instruction, the control part 145specifies a change in the time slot to be used (for example, use of timeslot TS3 is specified) and informs the line interfaces 141 and 142 ofthis change. At this time, the connection with the speech codec 146 viathe time slots TS1 and TS2 is released. In the above manner, it ispossible to execute the relaying and switching operation by means of themultiplexing bus 143.

A description will now be given of a second embodiment of the presentinvention with reference to FIG. 7. According to the second embodiment,the switching of a call to be relayed is carried out on the exchange110B as in the case of the structure shown in FIG. 1. As has beendescribed previously, the exchange 110B switches the 64 Kbps signals. Inthis regard, it is impossible to switch a call from the multiplexing bus143 to be relayed. With the above in mind, as shown in FIG. 7, there areprovided dummy bit inserting and dropping part 147 and two switches 148and 149. The two switches 148 and 149 are controlled by the control part145 in the following manner.

For example, when a call from the transmission path L1 should terminateat the terminal T2 accommodated by the exchange 110B, the line interface141 and the switch 148 are connected to each other by using time slotTS1 on the multiplexing bus 143. At this time, the controller 145 of theexchange 110B can recognize that the call should terminate at a terminalaccommodated by the exchange 110B. Then, the controller 145 controls theswitch 148 so that the connection between the multiplexing bus 143 andthe speech codec 146 is made, and controls the switch 149 so that theconnection between the speech codec 146 and the signal line 134 is made.Thus the 16 Kbps speech signals using time slots TS1 on the multiplexingbus 143 is converted into the 64 Kbps speech signal by the speech codec146. The 64 Kbps speech signal thus generated is output to the exchange110B via the switch 149 and the signal line 134.

Meanwhile when a call from the transmission path L1 should be related tothe transmission path L2, the control part 145 controls the switches 148and 149 so that the dummy bit inserting and dropping part 147. The 16Kbps signal from the multiplexing bus 143 is input to the dummy bitinserting and dropping part 147, which adds dummy bits equal to 48 Kbpsto the 16 Kbps signal. In this manner, pseudo-speech signal is sent tothe switching part 112 via the switch 149 and the signal line 134. Inresponse to an instruction from the control part 111, the switching part112 places the 64 Kbps pseudo-speech signal in a different time slot(for example, time slot TS4), and outputs it to the signal line 134. Thecontrol part 145 of the transmission device 120B is informed that thesignal to be relayed is located in time slot TS4 by the control part ofthe exchange 110B. Then the control part 145 controls the switches 148and 149 so that the dummy bit inserting and dropping part 147 isselected. The dummy bits equal to 48 Kbps are dropped from the 64 Kbpspseudo-speech signal by the dummy bit inserting and dropping part 147,and the remaining 16 Kbps speech signal is output of the multiplexingbus 143 via the switch 148. The line interface 142 receives, via timeslot TS4, the 16 Kbps speech signal to be relayed and outputs it to thetransmission path L2.

In the above manner different from the manner shown in FIG. 2, thetransmission device 120B does not execute the 64 Kbps PCM convertingoperation on the signal to be relayed even in the case where the relayand switching operation. Hence, it becomes possible to preventdeterioration of the quality of speech. Since the second embodiment doesnot execute the relaying and switching operation in the transmissiondevice 120B, a load of the transmission device 120B is smaller than thatin the first embodiment.

The embodiments of the present invention have been described. Thepresent invention is not limited to the communication system shown inFIG. 3.

INDUSTRIAL APPLICABILITY

The present invention is suitable for communication systems in which therelay and switching operation is executed using transmission pathshaving bit rates lower than or equal to 16 Kbps. Particularly, thepresent invention is suitable for small-size communication networks,such as a LAN and a VAN (Value Added Network).

We claim:
 1. A communication device comprising:a transmission deviceconnected to first and second transmission paths; andan exchangeconnected to said transmission device, wherein said exchangecomprises:first control means for determining whether an input signalwhich has a first bit rate and is received by said transmission devicevia the first transmission path is a first signal, which is to beterminated at one of a plurality of terminals connected to saidexchange, or a second signal, which is to be relayed to the secondtransmission path via said transmission device; and first switchingmeans for selectively connected said transmission device and saidplurality of terminals and transferring a signal having a second bitrate between them, and wherein said transmission device comprises:secondswitching means for selectively connecting said first and secondtransmission paths and said exchange and transferring a signal havingthe first bit rate between them; converting means for converting theinput signal having the first bit rate into a signal having the secondbit rate which is output to said exchange and for converting a signalwhich has the second bit rate and is received via said exchange into asignal having the first bit rate which is output to said secondswitching means; and second control means for controlling, when saidfirst control means determines that the input signal is the firstsignal, said second switching means so that the input signal is outputto said exchange via said converting means and for controlling, whensaid first control means determines that the input signal is the secondsignal, said second switching means so that the input signal is outputto the second transmission path without changing a bit rate thereof. 2.A communication device as claimed in claim 1, wherein said secondswitching means of the transmission device comprises a time switch whichexchanges time slots between the first and second transmission paths andsaid conversion means connected to said exchange.
 3. A communicationdevice as claimed in claim 1, wherein said second switching means of thetransmission device comprises a multiplexing bus which exchanges timeslots between the first and second transmission paths and saidconversion means connected to said exchange.
 4. A communication deviceas claimed in claim 1, wherein:said input signal comprises a speechsignal; and said conversion means comprises codec means for converting aspeech signal having the first bit rate into a speech signal having thesecond bit rate and for converting a speech signal having the second bitrate into a speech signal having the first bit rate.
 5. A communicationdevice as claimed in claim 1, wherein said first and second transmissionpaths comprises transmission paths which carry digital signals obtainedby multiplexing speech signals having bit rates lower than or equal to16 Kbps.
 6. A communication device as claimed in claim 1, wherein:saidtransmission device comprises an interface circuit connected between thefirst and second transmission paths and said second switching means; andsaid first control means of said exchange determines whether the inputsignal is the first signal or the second signal on the basis ofdiscrimination information contained in the input signal received viasaid interface circuit, said discrimination information indicatingwhether the input signal is to be terminated at one of the plurality ofterminals or is to be relayed to the second transmission path.
 7. Acommunication device as claimed in claim 1, wherein said first bit rateis lower than said second bit rate.
 8. A communication device as claimedin claim 1, wherein said transmission device comprises multiplexingmeans for multiplexing a plurality of signals which respectively havethe first bit rate and which are received via said conversion means andsaid first and second transmission paths to thereby output a multiplexedsignal to said first and second transmission paths and fordemultiplexing a multiplexed signal received via the first and secondtransmission paths into a plurality of signals respectively having thefirst bit rate and outputting these signals to said conversion means andsaid second switching means.
 9. A communication device as claimed inclaim 1, wherein said first and second transmission paths comprisedigital lines.
 10. A communication device comprising:a transmissiondevice connected to first and second transmission paths; andan exchangeconnected to said transmission device, wherein said transmission devicecomprises:codec means for converting an input signal which has a firstbit rate and is received via the first transmission path into a firstconverted input signal having a second bit rate and for converting anoutput signal which has the second bit rate and is received from saidexchange into a first converted output signal having the first bit rate;dummy bit inserting and dropping means for adding dummy bits to saidinput signal so that a second converted input signal having the secondbit rate is generated and for removing the dummy bits from a dummy-bitadded output signal received from said exchange so that a secondinverted output signal having the first bit rate is generated; andselecting means for selecting either said codec means or said dummy bitinserting and dropping means in accordance with a control signal,wherein said exchange comprises:first control means for determiningwhether the input signal which has the first bit rate and is received bysaid transmission device via said first transmission path is a firstsignal which is to be terminated at one of a plurality of terminalsconnected to said exchange or a second signal which is to be relayed tosaid second transmission path; and switching means for selectivelyconnecting said transmission device and said plurality of terminals totransfer signals respectively having the second bit rate, and whereinsaid transmission device comprises second control means for making saidselecting means select said codec means when said first control meansdetermines that said input signal is the first signal and for makingsaid selecting means select said dummy bit inserting and dropping meanswhen said first control means determines that said input signal is thesecond signal.
 11. A communication device as claimed in claim 10,wherein said input signal comprises a speech signal.
 12. A communicationdevice as claimed in claim 10, wherein said first bit rate is lower thansaid second bit rate.
 13. A communication device as claimed in claim 10,wherein said first and second transmission paths comprise transmissionpaths which transfer a digital signal obtained by multiplexing speechsignals respectively having bit rates lower than or equal to 16 Kbpswith each other.
 14. A communication device as claimed in claim 10,wherein said transmission device comprises multiplexing means formultiplexing a plurality of signals which respectively have the firstbit rate and which are received from said dummy bit inserting anddropping means and said codec means to thereby output a multiplexedsignal to said first and second transmission paths and fordemultiplexing a multiplexed signal received via the first and secondtransmission paths into a plurality of signals respectively having thefirst bit rate and outputting these signals to said codec means or saiddummy bit inserting and dropping means.